This invention relates to an electronic ballast and, in particular, an electronic ballast that imperceptibly modulates light output by interrupting power to one or more fluorescent lamps.
A fluorescent lamp is an evacuated glass tube with a small amount of mercury in the tube. The tube is lined with an adherent layer of a mixture of phosphors. Some of the mercury vaporizes at the low pressure within the tube and a filament or cathode sealed in each end of the tube is heated to emit electrons into the tube, ionizing the gas. A high voltage between the filaments causes the mercury ions to conduct current, producing a glow discharge that emits ultraviolet light. The ultraviolet light is absorbed by the phosphors and re-emitted as visible light. After the glow discharge terminates, the phosphors glow for a small but finite time known as persistence. Similarly, the glow discharge continues for an even smaller but finite time after power is removed.
A fluorescent lamp is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast connected to the lamp for limiting current.
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line into direct current (DC) and an inverter for changing the direct current into alternating current at high frequency, typically 25-60 kHz. Some ballasts include a boost circuit between the rectifier and the inverter.
Modern electronic ballasts perform the basic function of ballasting a fluorescent lamp significantly better than ballasts of just a decade ago in terms of power factor, efficiency, and the like. As typical with other electronic devices, electronic ballasts are now expected to perform an increasing number of additional functions. For example, many techniques have been proposed for dimming lamps by communicating over power lines or by communicating over a separate line to each ballast.
Other proposals, such as disclosed in U.S. Pat. No. 5,838,116 (Katyl et al.), include transmitting information from a fluorescent lamp by modulating the light from the lamp. The modulation described in the patent includes frequency modulation (FM) and amplitude modulation (AM). AM is obtained by interfering with the regulation of the boost circuit, thereby increasing the voltage of the high voltage rail in the inverter to increase light output momentarily.
Suitable photodetectors are a necessary part of the combination but are not described in detail in the Katyl et al. patent. Their existence and a variety of functions are merely attributed to certain blocks in a block diagram. It turns out that reliably detecting the modulation is not particularly easy. Interference from other light sources is a problem, as is signal to noise ratio in general. A strong, nearby signal tends to overload a detector and a weak, distant signal tends to become lost in noise.
Typically in the prior art, increased functionality is obtained only by increasing the complexity, and cost, of the ballast circuit. On the other hand, even if a particular function could be "free," it is inevitable that additional functions will be wanted. It is desired to provide those functions at minimal extra cost.
An advantage of digitally controlled dimmable ballasts is that the ballasts can be grouped for setting scenes or for locally brightening or dimming a part of a room. For this purpose, some dimming ballasts sold by Energy Savings, Inc. of Schaumburg, Illinois U.S.A. had an eight-way switch externally accessible on the ballasts. Depending on the setting of the switch, a ballast was assigned to one of eight possible zones. A scene could then have zone one at fifty percent of full brightness, zone two at seventy-five percent, and so on. This arrangement, while considerably better than changing the wiring in the building, still has the disadvantage of requiring physical access to the ballast.
It is possible to assign a unique number or identification (ID) to every ballast during manufacture. Each ballast can be addressed by ID and the control of scenes and zones can all be in one central unit. In theory, the ID would be written on a sticker and the sticker would be placed upon a fixture to show the ID. In practice, the chance of the ID being lost is quite high. Without any way to retrieve the information, the system capability would be lost or at least the fixture would have to be replaced.
In Europe, a system known as DALI, (digital addressable lighting interface) is being proposed. In this system, each ballast is given an ID at the factory and the ballasts are interrogated at the installation site to determine ID. After interrogation, the operator tells the controller to light up a first set of lamps and then the operator goes around the rooms and writes on a map where the lamps are lit. The process is repeated until all IDs are plotted on a map.
In view of the foregoing, it is therefore an object of the invention to provide a technique for modulating light output without additional circuitry in a digitally controlled electronic ballast.
Another object of the invention is to provide an electronic ballast that can identify itself readily by modulating the light output from one or more lamps coupled to the ballast.
A further object of the invention is to provide a detector for reliably converting modulated light from a fluorescent lamp into a series of pulses.
Another object of the invention is to provide two-way communication with a ballast with minimal additional circuitry.
A further object of the invention is to simplify the on-site identification of uniquely identified electronic ballasts.